We are studying the production of milk proteins by mammary tumors. Actively secreting cell cultures have been established from rat mammary tumor R3230AC. Casein is one of the proteins produced by these cells. A cell free RNA synthesis system has been developed which faithfully transcribes RNA product of all three RNA polymerases. We plan to study the synthesis of casein mRNA in nuclei from the cell cultures using as a probe a highly purified cDNA covalently attached to an oligo-dT cellulose column. The sensitivity of casein production to hormones will be determined. Clones producing large amounts of casein will be selected using immunochemical techniques.